what if ggplot guides were ggplots

ggguide.r

library(ggplot2)
library(grid)

d <- data.frame(x=rnorm(50), y=rnorm(50), f1 = rep(gl(5,5), 2), f2 = gl(2, 25))
p <- ggplot(d, aes(x,y, group=interaction(f1,f2))) +
  geom_line(aes(colour=f1, linetype=f2)) +
  geom_point(aes(colour=f1, shape=f1)) 


gb <- ggplot_build(p)
pp <- gb$plot

th <- theme_grey() + theme(legend.direction = "vertical")
gdefs <- ggplot2:::guides_train(scales = pp$scales, theme = th, guides = pp$guides, 
                      labels = pp$labels)

legs <- lapply(gdefs, "[[", "key")
names(legs) <- c("point", "segment", "point")


make_guide <- function(d, title){
  
  map <- list(as.symbol(colnames(d)[1]))
  names(map) <- colnames(d)[1]
  class(map) <- "uneval"
  
  ggplot(d, aes(x=0,y=0)) + facet_grid(.label~.) +
    geom_point(map) +
    scale_colour_identity()  +
    scale_shape_identity()  +
    scale_x_continuous(breaks=NULL)+
    scale_y_continuous(breaks=NULL) +
    labs(x=NULL, y=NULL) +
    theme_minimal() + 
    theme(panel.background = element_rect(fill="grey95"),
          legend.position = "none",
          strip.text.y = element_text(angle=0)) +
    ggtitle(title)
  
  
}

pl <- Map(make_guide, legs, names(legs))
pl <- lapply(pl, egg::set_panel_size, width=unit(1,"line"), height=unit(1,"line"))
gridExtra::grid.arrange(grobs = pl, ncol=1)

would require:

• ability to set panel sizes, align them (I think egg has the right concept with 3x3 frames, but might be biased)
• more orthogonal separation of tasks in ggplot_build, to train the scales on data independently of messy layout and theme business
• some clever NSE tricks for aesthetics and geom types
• a new function to merge guides
• some decisions on user interface and constraints
• think about colorbar scales

maybe easier to wrap it all in a separate package, kind of a stripped-down version of ggplot with no stats, coords, axes, guides.